This invention relates to light emitting diodes (LEDs) and, more particularly, to LEDs for use in lightwave systems.
In lightwave communication systems the light source is typically a diode laser or an LED which is coupled to an optical fiber. Information to be transmitted over the fiber is digitally encoded and used to pulse modulate the light intensity of the source. Depending upon the particular application, the bit rate of the system typically ranges from a few megabits per second to a few gigabits per second. In addition, the unrepeatered transmission path length of the system often dictates whether a laser or an LED is used. Lasers are usually found in long haul systems (e.g., hundreds of kilometers long) which require repeaters to regenerate and amplify the digital signal, whereas LEDs are common in short haul systems (e.g., a few tens of kilometers long or less) which do not require repeaters. One version of such short haul systems is known as an optical data link (ODL).
The design of lasers and LEDs for lightwave systems generally takes the form of a double heterostructure; e.g., an active, narrow bandgap layer sandwiched between and lattice-matched to a pair of opposite-conductivity-type, wider bandgap cladding layers. Under forward bias minority carriers are injected in the active region where they undergo radiative recombination to generate a light output at a wavelength related to the bandgap of the active layer. The direction of the light output depends on the device design. In lasers and edge-emitting LEDs light is extracted in a direction parallel to the active layer, and the output emerges from an end face (e.g., a cleaved facet) orthogonal thereto. On the other hand, in surface-emitting LEDs light is extracted in a direction perpendicular to the active region, and the output emerges from one of the cladding layers and/or from the substrate (if the latter is transparent at the wavelength of the generated light).
In order to couple light out of a surface-emitting LED, one major surface, say the bottom of the substrate, is typically provided with an annular electrical contact, and the opposite major surface is provided with a dot-shaped electrical contact in registration with the annulus. The dot contact restricts light emission to a correspondingly small area of the active region under the annulus, thereby enhancing the coupling efficiency, for example, to an optical fiber.
In common surface-emitting LEDs the dot contact is positioned in the center of the chip, which is typically a square parallelipiped. Illustratively, the major surfaces of the chip are about 22 mils square (480 square mils) and about 1700 chips are obtained from a one inch square wafer. Of course, the bigger the chip size, the fewer the number of chips which can be obtained from a given semiconductor wafer. Consequently, the unit cost of the chips is commensurately higher. At first blush it would appear that the straightforward solution to the problem is simply to scale down all of the dimensions of the device. This approach, however, is impractical because the annular contact also serves as a bonding pad for a wire bond. Reducing the chip size thus reduces the area of the bonding pad, making the wire bond operation a difficult and low yield process. Also, reducing the dot contact diameter increases the current density and hence degrades the reliability of the device.